Air ducting arrangement for combustion chambers



May 8, 1951 M. NATHAN AIR 'DUCTING ARRANGEMENT FOR comaus'rxou CHAMBERSPatented May 8, 1951 AIR DUCTING ARRANGEMENT FOR COMBUSTION CHAMBERSMatthew L. Nathan, Godalming, England, assignor to Power Jets (Researchand Development) Limited, London, England, a British company ApplicationMay 31, 1949, Serial No. 96,181 In Great Britain June 7, 1948 9 Claims.

This invention relates to improvements in combustion apparatus intendedto be particularly applicable to cases where combustion has to besupported by a fast-moving stream of gaseous fluid (hereinafterconsidered as being air).

The description fast-moving as applied to a combustion supporting airstream is intended herein to indicate that the mean speed of the airstream in its general direction of flow past a combustion initiatingzone, calculated from the ratio air volume passing in unit time/crosssectional area of the flow path, is suificiently high in relation to thespeed of flame propagation in the fuel-air mixture concerned to haveflame extinguishing properties. For hydrocarbon fuels burning in air thespeed of flame propagation is considered as being of the order of onefoot per second at an atmospheric temperature; the invention, on theother hand, is especially applicable to combustion apparatus forinternal combustion gas turbine and/or jet propulsion power units inwhich the speed of the air stream in its general direction of flow pasta combustion zone, calculated on the basis intended, might be from 10 to300 feet per second or even more depending on the design. The inventionis further of particular interest in relation to installations which, incommon with such power units, require stabilised burning to be supportednot only by a fastmoving air stream but also with high air/fuel ratios,and in which stable burning is required to be maintained at high ratesof fuel injection with a minimum of pressure loss.

The particular use of the invention at present envisaged is as a flametube suitable for use in a gas turbine aero engine and of the kindhaving entries at an upstream region for a part of an airstream toconstitute primary air, further entries located downstream of theprimary air entries for the entry of additional air to constitutesecondary and tertiary air, and a downstream outlet for the gases ofcombustion. Preferably, the primary air is swirled during its admissioninto the tube and the secondary air may also be swirled. Such a flametube has been described in our prior British specification No. 588,086.

It has been found that at high altitude it is beneficial to use a richmixture, i. e., with a relatively smaller quantity of primary air as forexample 15% of the total air flow. On the other hand this involves adecreased velocity of the gases through the flame tube, and as inaddition the density of the air is decreased relative to the density ofthe fuel droplets which are injected into the upstream end of the flametube, it may happen that a substantial part of the fuel is no longercarried along with the air flow within the tube, but tends to fall andbe deposited on the lowest surface of the flame tube to form a pool offuel thereon, and this is liable to interfere seriously with theefliciency of the combustion process.

Such a condition is especially liable to occur in the case of a wideflame tube, e. g., having a ratio of length to maximum diameter of lessthan 3:1, with low primary air admission, e. g., less than 15 per centof the total air flow.

Moreover while it is generally desirable to lead secondary air to anaxial region within the flame tube, the pipes, chutes or other deviceswhich have been used for this purpose are situated within the flame tubeand are exposed to the hot gases, and therefore frequently suffer suchcon siderable damage after a relatively short operating period as to becomparativley useless, thereby substantially diminishing the useful lifeof the apparatus.

The present invention seeks to minimise these disadvantages, and forthis purpose provides a flame tube of the kind set forth comprising,arranged coaxially within and located downstream of said primary airentries, a device in the form of two chambers arranged one Within theother of which the inner chamber has one or more openings for thepassage of gas from the outer chamber into the inner chamber and one ormore further openings for the passage of gas from the inner chamber intothe flame tube, and a plurality of ducts leading from the periphery ofthe flame tube into the outer chamber, the latter having one or moreopenings for the passage of gas from the outer chamber into the flametube.

Preferably the outer chamber has an open downstream end and the innerchamber has an open downstream end and an open upstream end. The twochambers may both be of circular section, and the upstream end of theannular space between them may be partly or wholly closed. In thepreferred form, the upstream ends of both chambers are of the samediameter, while the outer chamber is of frusto-conical form with itsnarrower end pointing downstream. Preferably also the inner chamber isin the form of a cone, having its apex'pointing downstream so that ineffect the device presents a frusto-conical chamber having an opendownstream end, and having its upstream end closed by a hollow conicalelement which enters into said chamber. With this arrangementair flowsfrom the periphery of the flame tube through the ducts into, the outerbe led via the entry 2a.

Firstly, it enables secondary air to be delivered from the periphery ofthe flame tube to an axial region thereof, while at the same timeproviding a large mass flow of relatively cooler air throughv thestructure so preventing it from becoming too hot.

Secondly, since the presence of th'ede'vice within the flame tubereduces the available cross sectional area of flow, the mean'velocity'ofthe air within the flame tube is increased. This promotes the movementof the fuel .cdroplets along with this air, thus preventing theabove-mentioned tendency of forming, in some conditions, a pool of fuelalong the lower surface of the flame tube.

The invention is illustrated by way of example in the accompanyingdrawing in-wh-ich, Figure 1 is 'a longitudinal section; Figure 2 is asection on the line IIII of Figure 1; Figure 3 is a sec- 'tion-ontheline III-III of Figure 1.

In Figure 1 a flame tube I having a closed-upstream conical end Ia andan open downstream end lb is disposedwithin an aircasing 2 so as tofor-man annular space 3 into which air can At the upstream end la of theflame .tube I is provided an axially disposed burner 4 by means of whicha stream of atomised liquid fuel is injected downstream into the flametube. In the wall of the conical portion la are entries in the form oftangentially tube l through its downstream outlet lb. A small amount ofsecondary air also enters through apertures lb.

The main entries for secondary air comprise a number of wide ducts 5extending into the space '3 and leading from this space into afrusto-conical chamber 6 within which is co-axially arranged a conicalchamber or baffle 1 having an open upstream end and an open downstreamend. The upstream end of chamber 6 abuts around the upstream end ofchamber? and the wall of chamber 6 is perforated with openings 6a.

With this arrangement the major part of the secondary air flows fromspace 3 through ducts :5 into the space 8 and thence through the opendownstream end of chamber 6 towards the outlet lb of the flame tube l. 7

Some of the air in the space 8 flows through aperture la into thechamber 1 and but through its upstream end lb and-is thence absorbed inthe reverse core of the vortex formed of the chutes lc.

Downstream of the entries for secondary air are provided three groups ofapertures le, If and lg whereby further air can pass from the space 3 towithin the flame tube I, all three groups of apertures being preferablydistributed symmetrically in a circumferential direction as a circularseries around the wall of flame tube 1. The apertures l eimmedi'atel'y"downstream of the chamber '5 are of relatively smaller diameter whilethe next succeeding group of apertures If in the downstreamdirection'are of greater diameter.

At its downstream outlet lb, the flame tube l is narrowed down to adiameter which is small relative to the diameter of the main body of thetube l, and it is general practice to narrow down the diameter of theend 2b of the casing in this region so as to form an annular space 9 ofvery small radial dimensions relative to space 3 and through which space9 a small proportion of the air stream passes for cooling purposes.

The small dimensions of this annular space 9 give rise however to astagnant region which does not provide suflicientcooling at the outletlb, and in order to avoid .this effect the flame tube wall I is providedin the immediately upstream vicinity of itsoutlet' lb, with a series ofapertures lg which are of still smaller diameter than the apertures 4eand are about the same diameter as aperture lh.

Preferably, matters are arranged so that approximately 10-15 per cent ofthe air stream enters the primary air entries lc approximately 20-40%enters the axial regionof the flametube through the entries Id and theremaining part of the air stream enters the flame tube l through thethree groups of apertures le, If and lg.

I claim:

l. A flame tube of circular section for the combustion of a highvelocity air stream, having means defining entries at an upstream regionof said tube for the admission of a part of said air stream into saidtube to constitute primary air, a hollow body of circular sectionarranged coaxially within said tube and downstream of said primary airentry defining means, a plurality of ducts for leading air from the.periphery of the flame tube to the interior of said body, a bafile ofcircular section arranged coaxially within said body at an upstreamregion'thereof and with its upstream face arranged to be exposed to adownstream r'low of combustion gases, the baffle wall having at leastone opening to admit a relatively minor flow of air from the interior ofsaid body to the upstream face of the bafiie, and said body having atleast one opening at its downstream region to admit a relatively majorflow of air from the interior of the body into the flame tube.

2. A flame :tube according to claim 1 wherein the bafile is of conical'formhaving its narrower end pointing downstream.

3. A flame tube according to claim '1 wherein the baffle fits within thebody in the region of the upstream end of said body.

4. A flame tube according to claim 1 wherein the baffle is of conicalform having its narrower end pointing downstream and fits within thebody in the region of the upstream end of said body.

5. A flame tube according to claim 1 wherein the body is of conical formhaving its narrower end open and pointing downstream and the bafile isof conical form having its narrower end pointing downstream, saidba'flie'fitting within the said body in the region of the upstream endof said body.

6. A flame tube according to claim '5 comprising towards the downstreamend of the flame tube two circumferentially axially'spaced series ofopenings for the admission of tertiary air, the more downstream seriesbeing of small diameter relative to themore upstream series.

'7. A flame tube according to claim 6 comprising a circumferentialseries of openings for secondary air in the wall of the flame tube andin the region of the ducts, the diameter of these openings being smallrelative to the diameter of 7 the more upstream openings for tertiaryair.

8. A flame tube according to claim 5 wherein the outer ends of the ductsproject beyond the wall of the flame tube and are inclined at an angleto the axis of the flame tube.

9. A flame tube according to claim 5 wherein the longitudinal wall ofthe body is perforated in the region of its upstream end to admit aminor flow of air from the interior of said body over said upstream end.

MATTHEW L. NATHAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

5 UNITED STATES PATENTS

